This invention relates generally to the field of satellite communication systems and, more particularly, to satellite-based RF communications.
An important issue in satellite communications systems is that of achieving a balance between desired satellite coverage requirements and the need for high gain beams to enable the system to achieve high data rates and high quality link performance with relatively small, low power, low cost user equipment (UE). The give and take needed to achieve a desirable balance between these system parameters is especially difficult in satellite systems intended to provide ubiquitous coverage and service, where the need to provide service throughout the entire satellite field of view (FOV) mitigates against the use of high gain beams each of which only covers a small area.
Consider the following example for illustrative purposes. A low earth orbit (LEO) satellite operating at an altitude of 780 kilometers (km) altitude with a 10 degree minimum service elevation angle has a FOV covering about 13.6 million square km. An antenna with 30 decibels of gain relative to isotropic radiation (dBi) typically has a half power beam width of 5.5 degrees, which covers approximately 4769 square km. Therefore, 2843 beams would be needed to cover the entire FOV with no overlap. Since, as a practical matter some overlap is necessary, the actual number of beams would approach 3000. If the antenna gain increases, for example to compensate for a higher altitude, the number of beams also increases. Thus, the use of high gain beams in a system with continuous ubiquitous coverage requires a complex satellite antenna system and a correspondingly complex radio subsystem to make use of all of the beams.
Most prior art systems have addressed the problem of high gain beam coverage in one of the following ways. Systems designed to provide ubiquitous coverage generally surrender high gain antenna beams and the corresponding advantages thereof. This allows the satellite to cover the FOV with a reasonable number of low gain beams, but places a severe burden on the UE to achieve the desired link performance. For example, if the 13.6 million square km FOV above is covered by 48 beams rather than thousands, the antenna gain drops to about 12 dBi resulting in an 18 dB deficit. This deficit is typically compensated for through increases in UE transmitter power and/or UE antenna size on the uplink and with UE antenna size and/or satellite transmitter power on the downlink. This solution is not attractive for systems that require small low cost UE.
An alternate prior art approach to avoiding the need for several thousand high gain beams is to relinquish simultaneous ubiquitous coverage. Referring back to the above example, if a satellite projects a 48 antenna beams with 30 dBi antenna gain in a 13.6 million square km FOV, less than 2% of the FOV is covered by the beams at any time. Generally prior art systems of this kind have simply not provided ubiquitous service. In these systems the locations where service will be provided is predetermined either permanently or for some period of time, and other areas are simply not served.
Other prior art systems using high gain beams have attempted to retain ubiquitous service by scanning or hopping the available beams over the FOV in a predetermined manner. This allows all desired locations to be served, but introduces serious capacity and delay limits that systems with continuous ubiquitous coverage do not have. Never-the-less, many prior art satellite systems, primarily those that operate in geostationary orbit (GSO) have accepted the limits of small predetermined spot beam coverage in order to avoid the satellite and UE complexities and costs associated with true ubiquitous coverage.
One way to use high gain spot beams in a satellite system and still provide ubiquitous service is to direct a spot beam at a location only when that location has traffic. This approach has been applied for the satellite to UE downlink in prior art systems. In these systems, the traffic includes the address of a destination UE that is associated by the satellite with the UE location. Thus, when and only when the satellite receives traffic destined for a particular location does the satellite direct a beam to that location. Once the traffic has been sent to the destination location, the directed beam is then available to direct traffic to different locations. Since data traffic for a particular user in packet switched networks generally comprises bursts of traffic interspersed with periods of idle time, this data directed beam steering approach avoids wasting precious satellite beam and radio resources in areas where there is no traffic to send.
In a prior art system described in U.S. Pat. No. 5,612,701 to Diekelman, a broad access beam is used to allow communication units to request assignment to a narrow spot beam. In that system, communication units are assigned both an uplink and a downlink spot beam for the duration of their connection to the satellite network. Diekelman teaches a method of steering a narrow spot beam to allow it to be simultaneously shared between a group of users located in close proximity of each other. This is an effective approach to resolving the trade between ubiquitous coverage and high gain beams for a voice telephony or other circuit switched network, but it would result in inefficient use of limited radio resources in a packet-switched data network.
The data directed beam steering approach used in some prior art systems for satellite downlinks enables a satellite to receive data from the network in a packet that contains the destination location. Since the satellite may read this destination information, it may determine where a downlink beam needs to be pointed in order to send traffic to the appropriate UE. However, this technique is not optimal for transmission over the UE to satellite uplinks, because the satellite does not known which UE desires to send traffic to the satellite at any particular time. Thus, the uplinks of prior art satellite systems have been limited to one of the less efficient methods of achieving ubiquitous service described above.
What is needed is a method and apparatus for efficiently accommodating varying demand for high gain beams on the UE to satellite uplink while minimizing satellite antenna and radio system complexity and power consumption in a ubiquitous coverage satellite system.